This invention relates to microwave power applicators, i.e., to devices for applying microwave power to workpieces. More particularly, this invention relates to microwave power applicators designed so as to substantially uniformly distribute hot spots which occur due to reflection standing wave patterns.
From a commercial point of view microwave energy usually is applied in one of three ways, namely resonant cavity ovens, waveguide applicators and horn type antenna applicators.
In resonant cavity ovens energy in the form of microwave radiation is fed into a totally enclosed metal box having dimensions suitable for supporting standing waves. The material to be heated is placed in the applicator and necessarily has smaller dimensions than those of the box. This type of applicator suffers from the disadvantages that heating is not even, pressure cannot be applied conveniently to the workpiece, and the workpiece cannot be added continuously to or removed continuously from the oven while it is operating.
In a waveguide applicator a slot is cut in the side of the waveguide in such a way that the workpiece may be introduced into the waveguide. Usually slots are provided on either side of the waveguide so that the workpiece may be passed through the waveguide continuously. This type of device is useful for heating thin webs of preformed material but cannot be used conveniently to apply pressure to the workpiece. In addition there is a limit to the size of the workpiece that can be passed through the slots, and devices of this type generally are not useful for heating thick workpieces.
In horn type antenna applicators a horn type antenna is used to convert the microwave energy from a transverse electric or magnetic mode to a free space or transverse electromagnetic mode. With this type of applicator it is difficult to control or contain the energy, and it is not possible to heat thick workpieces evenly or to apply pressure to the workpiece during heating.
In Canadian Pat. No. 836,140 issued Mar. 3, 1970 to Her Majesty in right of Canada as represented by the National Research Council of Canada there is disclosed a microwave power applicator having a basic configuration which resembles an embodiment of the instant invention. However, in the microwave power applicator disclosed in this patent no attempt is made to substantially uniformly distribute across the central tube which carries the workpiece hot spots which will occur as a result of reflection standing wave patterns within the central tube.